flow measurement part iv

FLOW MEASUREMENT PART IV

ER. FARUK BIN POYEN, Asst. Professor

DEPT. OF AEIE, UIT, BU, BURDWAN, WB, INDIA

[email protected]

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2Contents: Quantity Flow Meter Positive Displacement Meter Metering Pump

Mass Flow Meter Coriolis Mass Flow Meter

Miscellaneous Flow Meter Variable Reluctance Tacho generator Linear Resistance Element Flow Meter

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3Quantity Flow Meter

Quantity flow meter are of two types

1) Positive Displacement Meters and 2) Metering Pumps

Positive Displacement Meters have further sub divisions

Nutating Disk

Oscillating Piston

Rotating Vane

Lobed Impeller

Oval Gear

Reciprocating Piston

Metering Pumps come in these three types

Reciprocating Piston

Peristaltic Piston

Diaphragm Pump

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4Positive Displacement Flow Meters

Itis the only flow measurement technology that directly measures the volume of the fluid passing through the flow meter.

This is achieved by repeatedly entrapping fluid in order to measure its flow. This process can be thought of as repeatedly filling a bucket with fluid before dumping the contents downstream.

The number of times that the bucket is filled and emptied is indicative of the flow through the flow meter.

The rate at which it is passed is the volumetric flow rate. As because they pass a known quantity, they are ideal for certain fluid batch, blending

and custody transfer applications. They give very accurate information and are generally used for production and

accounting purposes.

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5Positive Displacement Flow Meters Advantages:

- Low to medium initial set up cost

- Can be used in viscous liquid flow Disadvantages

- Higher maintenance cost than other non-obstructive flow meters

- High pressure drop due to its total obstruction on the flow path

- Not suitable for low flow rate

- Very low tolerance to suspension in flow (particles larger than 100 µm need to be filtered before the liquid enters the flow meter)

- Gas (bubbles) in liquid could significantly decrease the accuracy

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6Nutating Disk Meter It has a moveable disk mounted on a concentric sphere located in a spherical side-walled

chamber. The pressure of the liquid passing through the measuring chamber causes the disk to rock in a

circulating path without rotating about its own axis. It is the only moving part in the measuring chamber. A pin extending perpendicularly from the disk is connected to a mechanical counter that

monitors the disk's rocking motions. Each cycle is proportional to a specific quantity of flow. As is true with all positive-displacement meters, viscosity variations below a given threshold

will affect measuring accuracies.

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7Nutating Disk Meter

Advantages:

Relatively low cost

Applicable to automatic liquid batching system

Make use of moderate pressure loss

Construction available in several material

Disadvantages

Limited to pipe size and capacity

Accuracy average

Not suitable for slurries

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8Oscillating Piston It is similar to that of a nutating disc except that the measurement device

is a split ring oscillating in only one plain. It comprises a slotted cylinder oscillating about a dividing bridge which

separates the inlet and outlet ports. Initially the piston rests at the neutral position. As fluid enters the section, the ring starts rotating from left to right until

the fluid is escorted to the outlet. The rotation of piston is transmitted through a diaphragm to the gear

train and register. This type is suitable for viscous and corrosive liquids. Accuracy is ±

1%.

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9Oscillating Piston

Advantages

Good accuracy at low flow rates

Good repeatability

Easy installation

Moderate cost

Disadvantages

Only small sizes are available

Limited power capacity

Fluid should be clean

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10Rotating Vane Meter Spring loaded vanes slide in and out of a channel in a rotor so that they

make constant contact with the eccentric cylinder wall. When the rotor turns, a known volume of fluid is trapped between the

two vanes and the outer wall. The flow rate is based on volume per revolution. The piston type is suitable for accurately measuring small volumes and

is not affected by viscosity. Limitations with this device are due to leakage and pressure loss.

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11Rotating Vane Meter Advantages

- Reasonable accuracy of 0.1%.

- Suitable for high temperature service, up to 180°C

- Pressures up to 7 Mpa

- Maximum flow rate that it can support is 17500 gpm

Disadvantages

- Suitable for clean liquids only

- High cost

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12Lobed Impeller This type of meter uses two lobed impellers, which are geared and

meshed to rotate at opposite directions within the enclosure. A known volume of fluid is transferred for each revolution.

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13Lobed Impeller

Advantages- High operating pressures, up to 8Mpa.

- High temperatures, up to 200°C.

- Accuracy ± 0.1 % to ± 0.5 %

Disadvantages- Poor accuracy at low flow rates.

- Bulky and heavy.

- Expensive

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14Oval Gear

Two oval gears are intermeshed and trap fluid between themselves and the outer walls of the device.

The oval gears rotate due to the pressure from the fluid and a count of revolutions determines the volume of fluid moving through the device.

The viscosity of the fluid can affect the leakage, or slip flow. If the meter is calibrated on a particular fluid, it will read marginally higher should the

viscosity rise. Newer designs of this type of meter use servomotors to drive the gears. This eliminates the pressure drop across the meter and also the force required to drive the

gear. This eliminates the force, which causes the slip flow. This mainly applies to smaller sized

meters and significantly increases the accuracy at low flows.

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15Oval Gear Advantages

- High accuracy of 0.25%

- High operating pressures, up to 10MPa

- High temperatures, up to 300°C

- Wide range of materials of construction

Disadvantages

- Pulsations caused by alternate drive action

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16Reciprocating Piston Meter This type is mainly used for heavy chemical and manufacturing fluids. It employs a piston with inlet and outlet check valves with the piston moving in a

reciprocating manner. Check valves prevent back flow. As piston retracts from cylinder, fluid is filled in. as piston re-enters, the fluid is

forcefully discharged out of the cylinder. It has a very high temperature sustenance capability of 540 ° C and can handle 100000

psig pressure. Accuracy ranges between ± 0.5 % and 1 %.

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17Reciprocating Piston Meter

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18Metering Pump A metering pump is a positive displacement pump which is used to provide a

predictable and accurate rate of process fluid flow. Any positive displacement pump may be used as a metering pump due to its

volumetric mode of fluid transfer. Only those pumps which have very little internal or external leakage are used for as

metering pumps. Metering Pumps are of three types 1) Reciprocating piston Pump 2) Peristaltic Pump 3) Diaphragm Pump

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19Reciprocating Piston Pump

It is mainly used in heavy chemical and manufacturing industry employing a piston or a plunger having inlet and outlet check valves and a piston moving with a reciprocating motion in the chamber. Check valves prevent back flow.

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20Peristaltic Pump It is a type of positive displacement pump used for pumping a variety of fluids. The fluid is

contained within a flexible tube fitted inside a circular pump casing (though linear peristaltic pumps have been made).

A rotor with a number of "rollers", "shoes", "wipers", or "lobes" attached to the external circumference of the rotor compresses the flexible tube. As the rotor turns, the part of the tube under compression is pinched closed (or "occludes") thus forcing the fluid to be pumped to move through the tube.

Additionally, as the tube opens to its natural state after the passing of the cam ("restitution" or "resilience") fluid flow is induced to the pump. This process is called peristalsis.

There will be two or more rollers, or wipers, occluding the tube, trapping between them a body of fluid.

The body of fluid is then transported, at ambient pressure, toward the pump outlet. Peristaltic pumps may run continuously, or they may be indexed through partial revolutions to deliver smaller amounts of fluid.

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21Peristaltic Pump

22Diaphragm Pump

A diaphragm pump (also known as a Membrane pump, Air Operated Double Diaphragm Pump (AODD) or Pneumatic Diaphragm Pump) is a positive displacement pump that uses a combination of the reciprocating action of a rubber, thermoplastic or teflon diaphragm and suitable valves on either side of the diaphragm (check valve, butterfly valves, flap valves, or any other form of shut-off valves) to pump a fluid.

When the volume of a chamber of either type of pump is increased (the diaphragm moving up), the pressure decreases, and fluid is drawn into the chamber.

When the chamber pressure later increases from decreased volume (the diaphragm moving down), the fluid previously drawn in is forced out.

Finally, the diaphragm moving up once again draws fluid into the chamber, completing the cycle.

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23Diaphragm Pump

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24Mass Flow Meter Coriolis Mass Flow meter: The basis of the Coriolis meter is Newtons’ Second Law of

Motion, where: Force = Mass x Acceleration

The conventional way to measure the mass of an object is to weigh it. In weighing, the force is measured with a known acceleration (9.81m/sec^2).

This type of measuring principle is not easy or possible with fluids in motion, particularly in a pipe.

It is possible to manipulate the above formula and apply a known force and measure, instead, the acceleration to determine the mass.

The Coriolis Effect causes a retarding force on a rotating section of pipe when flow is moving outward, conversely producing an advance on the section of pipe for flow moving towards the axis of rotation.

When the full section of pipe is moved about its axis in an oscillatory motion, the outgoing section of pipe is retarded (or decelerated) and the return section is advanced (or accelerated), producing a twist in the pipe.

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25Coriolis Meter

The force is applied to oscillate the flow pipes and the Coriolis Effect is the principle used to determine the acceleration due to the torque (the amount of twisting).

Sensors are used to measure the amount of twist in the flow tubes within the meter as a result of the flow tube vibration and deflection due to the mass flow.

The amount of twist measured is proportional to the mass flow rate and is measured by magnetic sensors mounted on the tubes.

Developments on the looped pipe Coriolis meter were made to keep to the pipes straight.

This is done by making the pipes straight and parallel. The force is applied by oscillating the pipes at the resonant frequency. This has the advantage of reducing pressure loss in the pipeline.

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26Coriolis Meter

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27Coriolis Meter Advantages- Direct, in-line mass flow measurement.- Independent of temperature, pressure, density, conductivity and viscosity.- Sensor capable of transmitting mass flow, density and temperature information.- High density capability.- Conductivity independent.- Suitable for hydrocarbon measurements.- Suitable for density measurement. Disadvantages- Cost.- Affected by vibration.- Installation costs.- Adjustment of zero point.

28Miscellaneous Type Flow Meters:Variable Reluctance Tachogenerator It is used for measurement of linear and angular velocity measurement. Magnetomotive force (mmf) is the force that causes flux to be established and it is

analogous to the electromotive force for electric circuits. SI unit of mmf is Ampere and it only refers to one turn of a coil. The opposition to the establishment of magnetic flus is reluctance.

Where mmf is in Ampere turns and ϕ (flux) is in Weber.

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29Variable Reluctance Tachogenerator If the time – varying flux ϕ is linked by a single trun of coil, then the back emf

developed in the coil can be expressed as The permanent magnet is extended by a soft iron pole piece. The teeth of the wheel

move is close proximity to the pole piece. Therefore, the flux linked by the coil changes with time and voltage is developed across the coil. The total flux (ϕ T) linked by the coil of m turn is expressed as

Again it is known, with reluctance being minimum, flux becomes maximum and vice versa. The variation of ϕ T with angular position θ is expressed as

α = mean flux, β = time – varying flux’s amplitude, n = no. of teeth of the wheel.

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30Variable Reluctance Tachogenerator

The induced emf is expressed as

Again and

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31Miscellaneous Type Flow Meters:Linear Resistance Element Flow Meter For a small flow rate or for highly viscous flows the linear resistance element flow

meter (also called capillary flow meter) is effectively suitable. It is a constant head loss type and its principle of operation is based on Hagen –

Poiseulle equation for laminar flow in tubes.

Q = flow rate D = inside diameter L = length of tube μ = viscous coefficient (p1 – p2) = pressure drop along tube length

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32Linear Resistance Element Flow Meter The total metered fluid is guided by means of flow straighteners to the metering element in the shape

of a bundle of capillary tubes in honeycomb configuration. Due to high viscosity, small flow rate and small size of diameter tube, the Reynold’s number is small

and is in the laminar range of flow. The primary advantage of this flow meter is that flow rate is directly proportional to the pressure

drop and that is why it is termed as linear resistance element flow meter. Advantages Accurate average measurement Good damping ability Reverse flow is measurable Disadvantages Subject to plugging for slurries fluid High pressure loss involved Expensive

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33References: Chapter 11: Flow Measurement, “Industrial Instrumentation and

Control” by S K Singh. Tata McGraw Hill, 3rd Edition. 2009, New Delhi. ISBN-13: 978-0-07-026222-5.

Chapter 12: Flow Measurement, “Instrumentation, Measurement and Analysis”. 2nd Edition, B C Nakra, K K Chaudhry, Tata McGraw-Hill, New Delhi, 2005. ISBN: 0-07-048296-9.

Chapter 7: Flowmeter, “Fundamentals of Industrial Instrumentation”, 1st Edition, Alok Barua, Wiley India Pvt. Ltd. New Delhi, 2011. ISBN: 978-81-265-2882-0.

Chapter 5: Flow Measurement, “Principles of Industrial Instrumentation”, 2nd Edition. D. Patranabis, Tata McGaw-Hill, New Delhi, 2004. ISBN: 0-07-462334-6.

flow measurement part iv

Engineering